E — Make it Palindrome
(https://atcoder.jp/contests/abc290/tasks/abc290_e)
- Please refer the editorial
( https://atcoder.jp/contests/abc290/editorial/5809 )
Can anyone please prove the given formula that is used to calculate total number of lines ? 
tagging top contributors for better reach awoo, -is-this-fft-, nor, IceKnight1093
My reply is a bit off-topic, but I find the user editorial a bit easier to understand. You can google translate it, but just by looking at the Python code, it is easier to understand than the official editorial.